Conventional steering systems are now replete with power-assisted mechanisms in which auxiliary power sources assist a driver by contributing to the force required to direct dirigible road wheels. Power-assisted systems typically include a power steering cylinder which cooperates with a piston to define a pair of opposed working chambers. The steering system shown in U.S. Pat. No. 4,063,490 dated Dec. 20, 1977, which is assigned to the assignee of this invention, is illustrative and is incorporated by reference. As disclosed therein, the piston is carried on a piston rod that is joined to or is integral with a gear rack, which engages a pinion. A rotary valve mechanism is used to control the distribution of pressure to the working chambers to provide a powered assist to the manual steering effort applied to the pinion through a steering wheel. Located in a power steering valve housing, the rotary valve structure includes a valve sleeve having internal valve lands and an internal rotary valve spool having external valve lands that register with the internal valve lands. The spool is connected to a driver-controlled input shaft. Linking the input shaft and the pinion is a torsion bar which deflects in response to the magnitude and direction of torque applied to the steering wheel. The valve sleeve is joined to the pinion for common rotary movement. When such movement occurs, the external lands of the valve spool are displaced relative to the internal lands of the valve sleeve. This displacement controls distribution of pressurized fluid which is ducted to the valve sleeve through passages, thence to a working chamber of the power steering cylinder. The magnitude of the pressure differential across the fluid piston is determined by the magnitude of the torque which, in turn, determines the relative position of the internal and external valve lands.
The pumps used in such power steering systems are generally positive displacement pumps that deliver a constant flow through the rotary power steering valve. The valve lands of the spool and the sleeve are arranged so that flow occurs when the valve lands are positioned in a centered relationship. This is known as an open-center valve system that distributes pressure to the left turn working chamber or the right turn working chamber and gradually closes the return flow path into the fluid reservoir, depending on the magnitude and direction of the torque applied to the torque input shaft. Such systems are disclosed, for example, in U.S. Pat. No. 4,516,471, which is also assigned to the assignee of the present invention and is hereby incorporated by reference.
Driver preferences have evolved to the point where such steering systems should function without diminishing "road feel." "Good" road feel is a condition wherein a steering wheel self-centering force builds up gradually and smoothly from an off-center position in a safe, reassuring way. In such a condition, a vehicle displays good "directional sense" and "directional stability," yet the driver feels he is in command at all times. More road feel is obtained with more manual effort. Correspondingly, less road feel is transmitted to the driver with more power assist because high power gain resists road forces feeding back to the driver via the steering wheel.
If a given vehicle is difficult to steer manually in certain driving conditions, it requires more power assist. For example, the conventional vehicle is often harder to steer manually at low speeds, during cornering, and during parking. Power assist may then be available, but it should decrease with increasing vehicle speed.
To meet challenges posed by a firm, stable feel in steering systems at highway speeds while offering power assistance in low speed operations such as parking, the concept of speed-sensitive power steering has evolved in various forms. Such systems provide a firm stable "steering feel" with high driver steering efforts at high speeds, and a significant easing of manual effort as vehicle speed decreases. Speed-sensitive and variable assist power steering systems are disclosed, for example, in U.S. Pat. Nos. 4,434,866; 4,561,521; and 4,570,736, each also being assigned to the assignee of this invention.
Nevertheless, even these speed-sensitive power steering systems leave unsatisfied the preferences of today's discerning driver for compensation of pull and drift which arise when a crosswind is encountered, when the road is cambered, when the front tires have unequal driving characteristics, or when the front wheels are misaligned. On a long highway trip, a constant steering wheel pull that is required to keep the vehicle along a desired path is tiring and annoying. My invention makes it possible to avoid driver fatigue and irritation caused by a continuous pull exerted on the steering wheel. The invention enables the effort required of a driver to keep the vehicle along a desired path to be partly or completely trimmed out. Such a feature reduces the amount of manual effort required to be exerted over time. Additionally, my invention provides an improvement embodying these features which is adaptable for use with conventional power steering, as well as with variable assist systems.
Since individual driver preferences vary widely, it would be desirable to accommodate such preferences by providing a compensation feature in power steering systems which would maintain a residual amount of force required of the driver to maintain the vehicle's path in the desired direction.
Because good "feel" for corrective steering is a matter of subjective taste, it would be useful to provide a manual override to an automated power steering trim or compensation system so that the driver can adjust the desired amount of manual effort he or she prefers to apply to the steering wheel in order to maintain the vehicle along the desired path.
Additionally, an optimal compensation system would progressively offset the amount of manual effort exerted on the steering wheel over time, so as to make force diminution barely perceptible to the driver and introduce no sudden changes.